Data signal quality evaluation method and apparatus using high speed sampling

ABSTRACT

The scale and complexity of an apparatus is reduced by omitting a clock extraction section. The apparatus includes: a sampling pulse train generation device which generates an optical or electrical sampling pulse train, independently of an input optical or electrical data signal with a bit rate f 0  (bit/s), and which has a repetition frequency f 1  (Hz); a data signal sampling device which samples the data signal in accordance with the sampling pulse train to obtain a sampled signal; a voltage retaining device which converts the sampled signal, and stores pieces of electrical digital data; an electrical signal processing device which reads the digital data at once or sequentially to obtain a signal eye-diagram and evaluates optical data signal quality parameters; and a trigger signal generation device which applies triggers indicating the start/finish of data acquisition and data read to the voltage retaining device and the electrical signal processing device. respectively.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a data signal quality evaluationmethod using high speed sampling, suitable for use when sampling anoptical or electrical data signal with a predetermined bit rate anddisplaying an eye-diagram and measuring signal quality. 2. Descriptionof the Related Art

[0003] A first example of a conventional optical signal qualityevaluation apparatus is shown in FIG. 10 (see reference document“Handbook of ELECTRONIC TEST EQUIPMENT (Section 5-8. SAMPLINGOSCILLOSCOPE), pp. 184-189, JOHN D. LENK, Prentice-Hall, Inc., EnglewoodCliffs, N.J., 1971”, reference document “Modeling of the HP-1430AFeedthrough Wide-Band (28-ps) Sampling Head, SEDKI M. RIAD, IEEETransactions on Instrumentation and Measurement, Vol. 1M-3 1, No. 2,June 1982, pp. 110-115”, for example). This conventional optical signalquality evaluation apparatus comprises an opto-electric conversiondevice 101 which converts an optical signal with a bit rate of f₀(bit/s) into an electric intensity modulated signal, a clock extractiondevice 102 which extracts a clock from the electric intensity modulatedsignal, a sampling clock generation device which generates a samplingclock with a repetition frequency of f₁(Hz) (f₁=(n/m)f₀+a, where n and mare natural numbers, and a is the offset frequency) synchronized withthe clock extracted by the clock extraction device 102, and anelectrical signal processing device 104. The electrical signalprocessing device 104 samples the electric intensity modulated signalinput via the clock extraction device 102 in accordance with thesampling clock, and displays a sampled data distribution sequentiallybased on the obtained sampled electrical signal, thereby obtaining asignal eye-diagram, and evaluates the optical signal quality parameters.

[0004] As a second conventional example, resembling the firstconventional example described above, there are optical sampling devicesusing a sampling optical pulse train having a repetition frequency of f₁(Hz) (f₁(n/m).f₀+a, where n and m are natural numbers and a is theoffset frequency) and a pulse width sufficiently narrower than atimeslot of an optical signal, and optical sampling devices using asampling clock (see Japanese Patent No. 2677372, Japanese Patent No.3239925, reference document “100 Gbit/s optical signal eye-diagrammeasurement with optical sampling using organic nonlinear opticalcrystal, H. Takara, S. Kawanishi, A. Yokoo, S. Tomaru, T. Kitoh and M.Saruwatari, Electronics Letters, Vol. 32, No. 24., 21st November 1996,pp. 2256-2258”, for example). These optical sampling devices areprovided before the opto-electric conversion device. In these examples,an optical splitter splits the optical signal, and optical sampling isperformed using a sampling clock or a sampling optical pulse trainsynchronized with the clock obtained by performing clock extraction fromone of the split optical signals. The sampled optical signal is thenconverted into a sampled electrical signal by the opto-electricconversion device. The electrical signal processing device then displayssequentially a sampled data distribution based on the obtained sampledelectrical signal, thereby obtaining a signal eye-diagram, and evaluatesthe optical signal quality parameters.

[0005] The repetition frequency of the sampling clock in the firstconventional example is normally within the range of several dozen toseveral hundred kHz, and it takes time to obtain a signal eye-diagramwhich is necessary and sufficient for evaluation, and therefore wanderof the optical signal becomes an issue. Consequently, clock extractionwas essential. In the second conventional example in which opticalsampling is performed by the sampling clock or the sampling opticalpulse train, the repetition frequency of the sampling clock or thesampling optical pulse train is approximately 10 MHz, but because it isnecessary to perform the electrical signal processing to determine thesampled data distribution sequentially, the effective sampling ratedecreases, and it takes time to obtain a necessary and sufficient signaleye-diagram for evaluation, and therefore wander of the optical signalbecomes an issue. Consequently, clock extraction was essential.

[0006] As described above, because clock extraction is required in boththe first and second conventional examples, this presents such problemsas increases in the scale of the apparatus, increases in the complexityof the method or apparatus, and increases in the cost of the apparatus.An optical signal monitoring apparatus using asynchronous sampling (seeEuropean Patent Application Publication No. EP 0920150 A2, referencedocument “Optical signal quality monitoring method based on opticalsampling, I. Shake, H. Takara, S. Kawanishi and Y. Yamabayashi,Electronics Letters, Vol. 34, No. 22, 29th October 1998, pp. 2152-2154”,for example), is proposed as a third conventional example, as an opticalsignal monitoring apparatus which does not require clock extraction.However, because this method evaluates an optical signal intensitydistribution based on an asynchronous eye-diagram, it still cannot beapplied to degradation in the time domain (such as jitter andpolarization dispersion).

SUMMARY OF THE INVENTION

[0007] In accordance with the above circumstances, an object of thepresent invention is to provide a data signal quality evaluation methodand apparatus which by omitting a clock extraction section, enables thescale of the apparatus to be reduced, which allows the method or theapparatus to be simplified, and which allows the cost of the apparatusto be reduced, and which is capable of monitoring not onlysignal-to-noise ratio (SNR) degradation and wavelength dispersiondegradation, but also signal quality degradation in the time domain,such as jitter and polarization dispersion degradation.

[0008] In order to achieve the above objects, the data signal qualityevaluation method of the present invention comprises the steps of: afterrepeating N times (where N is a natural number) a process in which aninput optical or electrical data signal with a bit rate of fo (bit/s) issampled using an optical or electrical sampling pulse train, which isgenerated independently of the data signal, and which has a repetitionfrequency f₁ (Hz) which differs from the bit rate f₀ (bit/s), a thusobtained optical or electrical sampled signal is converted into a pieceof electrical digital data, and the piece of the electrical digital datais stored in a buffer, reading N pieces of the electrical digital datastored in the buffer at once or sequentially and performing electricalsignal processing of the N pieces of the electrical digital data, toobtain a signal eye-diagram and to perform data signal waveformmeasurement and quality evaluation.

[0009] Furthermore, the data signal quality evaluation apparatus of thepresent invention comprises a sampling pulse train generation devicewhich generates an optical or electrical sampling pulse train,independently of an input optical or electrical data signal with a bitrate of f₀ (bit/s), and which has a repetition frequency f₁ (Hz) whichdiffers from the bit rate fo (bit/s); a data signal sampling devicewhich samples the data signal with the bit rate f₀ (bit/s) in accordancewith the sampling pulse train to obtain a sampled signal; a voltageretaining device which converts the sampled signal which is an opticalor electrical sampled signal obtained by means of the data signalsampling device into a piece of electrical digital data, and stores aplurality of pieces of the electrical digital data; an electrical signalprocessing device which reads the plurality of pieces of the electricaldigital data stored in the voltage retaining device at once orsequentially to obtain a signal eye-diagram and to evaluate optical datasignal quality parameters; and a trigger signal generation device whichapplies triggers indicating the start/finish of data acquisition to thevoltage retaining device, and applies triggers indicating thestart/finish of data read to the electrical signal processing device.

[0010] The present invention can provide a data signal qualityevaluation method and apparatus, in which a reduction in the scale ofthe apparatus, simplification of the method and the apparatus, and areduction in the cost of the apparatus can be achieved due to the factthat a clock extraction section is omitted, and in which by acquiringdata using high speed sampling and a buffer, a pseudo-synchronizedoptical signal eye-diagram unaffected by wander can be obtained despitethe lack of a synchronizing device, and which can monitor not onlysignal-to-noise ratio (SNR) degradation and wavelength dispersiondegradation, but also signal quality degradation in the time domain,such as jitter and polarization dispersion degradation.

[0011] Furthermore, by using an optical sampling method, the presentinvention can be applied to a wider range of optical signal bit ratesthan methods in which electrical sampling methods are used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram showing the construction of a firstembodiment of the present invention.

[0013]FIG. 2 is a block diagram showing the construction of a secondembodiment of the present invention.

[0014]FIG. 3 is a block diagram showing the construction of a thirdembodiment of the present invention.

[0015]FIG. 4 is a diagram showing an example of data signal qualityevaluation according to the embodiments shown in FIG. 1 to FIG. 3.

[0016]FIG. 5 is a diagram showing another example of data signal qualityevaluation according to the embodiments shown in FIG. 1 to FIG. 3.

[0017]FIG. 6 is a diagram showing another example of data signal qualityevaluation according to the embodiments shown in FIG. 1 to FIG. 3.

[0018]FIG. 7 is a diagram showing another example of data signal qualityevaluation according to the embodiments shown in FIG. 1 to FIG. 3.

[0019]FIG. 8 is a diagram showing another example of data signal qualityevaluation according to the embodiments shown in FIG. 1 to FIG. 3.

[0020]FIG. 9A through FIG. 9D are diagrams showing samples of dataplotting according to the embodiments shown in FIG. 1 to FIG. 3.

[0021]FIG. 10 is a block diagram showing the construction of aconventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] First Embodiment

[0023] A first embodiment of a data signal quality evaluation apparatusaccording to the present invention is shown in FIG. 1. This embodimentis in accordance with the invention as disclosed in claim 10. In thepresent embodiment, an electrical signal electrical sampling device 12is used as the data signal sampling device. In the case of the presentembodiment, the input data signal is an electrical data signal, and asampling clock generation device 13 is used as the sampling pulse traingeneration device. FIG. 1 shows a specific case in which an opticalsignal with a bit rate f₀ (bit/s) is converted into an electricintensity modulated signal via an opto-electric conversion device 11 andthen input into the electrical signal electrical sampling device 12.However, in a case in which an electrical signal with a bit rate f₀(bit/s) is input directly into the electrical signal electrical samplingdevice 12, the opto-electric conversion device 11 is unnecessary, andsuch an embodiment is also included in the present embodiment. Theoperation of the present embodiment is described below.

[0024] The optical signal with a bit rate f₀ (bills) arrives at theelectrical signal electrical sampling device 12 via the opto-electricconversion device 11 as an electric intensity modulated signal. Inaddition, a sampling clock is generated by the sampling clock generationdevice 13 at a repetition frequency of f₁(Hz) (f₁(n/m) f₀+a, orf₁=(n/m)f₀−a, where n and m are natural numbers, and a is the offsetfrequency). The electrical signal electrical sampling device 12 samplesthe electric intensity modulated signal in accordance with this samplingclock to obtain a sampled electrical signal. A voltage retaining device14 performs analog/digital conversion (A/D conversion) of the sampledelectrical signal according to trigger signals from a trigger signalgeneration device 15 indicating the start of data acquisition, and atemporary memory storage operation is performed. The voltage retainingdevice 14 stores a plurality of pieces of sampled data for the timeuntil a trigger signal indicating the finish of data acquisition is sentfrom the trigger signal generation device 15. The plurality of pieces ofthe sampled data are then output according to a trigger signal from anexternal source. Here for example, a high-speed A/D conversion circuitwhich comprises an electrical buffer memory with a capacity equal to orgreater than a kilobyte and which has the function of MHz to GHzsampling could be used. Furthermore, the sampling gate width of thesampling clock is preferably less than ¼ or thereabouts of the timedetermined by the reciprocal of the bit rate f₀ of the optical signal.

[0025] After data acquisition has been performed for a fixed period oftime and a plurality of pieces of sampled data have been stored in thevoltage retaining device 14, the trigger signal generation device 1-5sends a trigger signal indicating the start of data read to anelectrical signal processing device 16. Then the electrical signalprocessing device 16 reads the plurality of pieces of the sampled datafrom the voltage retaining device 14 according to the trigger signalindicating the start of data read, obtains a signal eye-diagram from theplurality of pieces of the sampled data, and performs such functions asdisplaying the signal eye-diagram and performing predeterminedarithmetic processing relating to signal-to-noise ratio (SNR)degradation, wavelength dispersion degradation, and signal qualitydegradation in the time domain such as jitter and polarizationdispersion degradation and displaying the results of the predeterminedarithmetic processing so that the optical signal quality parameters canbe evaluated, or outputting the results to a predetermined externaldevice.

[0026] Here, the repetition frequency f₁ of the sampling clock isdetermined based only on the number (n/m)f₁ which is related to theoptical signal bit rate f₀, and is not made to follow the bit phase ofthe optical signal using clock extraction or the like. For example, herea case in which the optical signal bit rate is one of either 2.5 Gbit/A,10 Gbit/s or 40 Gbit/s is considered. In this case, if 100 MHz which isone of the common measures of these bit rates is known as theinformation required to determine the repetition frequency of thesampling clock, f₁ can be determined based on this figure. For example,if the repetition frequency of the sampling clock is set to (100 MHz+aHz), and it is assumed that 15000 pieces of sampled data are required,then a data acquisition time of approximately 150 μs is required. Inother words, in this method, the only bit phase shift caused by wanderwhich affects the eye-diagram used in the evaluation is the variationwithin approximately 150 μs. If the temperature difference within oneday is 60° C. (over 12 hours), then the temperature variation within 150μs is approximately 2.1×10 ⁻⁷° C. Taking into consideration the factthat the maximum amount of pulse delay in a nylon coated quartz fiberwhich forms the transmission line of the optical signal is approximately0.2 ps/m/° C. (actual measured value), the amount of pulse delay causedby variation in the temperature of the whole transmission line of 100knm according to air temperature variation is 4.2×10⁻³ ps within 150 μs.Because this value is small enough to be ignored not only in electricalsampling at a resolution of approximately 20 μs but even in opticalsignal optical sampling at a resolution of approximately 1 ps, theeye-diagram according to the present method can be evaluated as if it isa synchronous eye-diagram.

[0027] An example of a suitable range for setting the offset frequency ais${\frac{( \frac{n}{m} )^{2}q}{k + {( \frac{n}{m} )q}}f_{0}} \leq a < {\frac{( \frac{n}{m} )^{2}q}{k - 1 + {( \frac{n}{m} )q}}f_{0}}$

[0028] (where k and q are natural numbers, for example.

[0029] Here, what n/m, k, and q refer to is described in detail in the“display examples of eye-diagrams according to the embodiments” below.

[0030] Furthermore, the input signal (data signal) in the data signalquality evaluation apparatus of the present invention is not limited toan optical signal with a bit rate f₀ (bit/s) as described above, and maybe an electrical signal with a bit rate f₀ (bit/s). In such a case, inthe embodiment shown in FIG. 1, the opto-electric conversion device 11may be omitted, and the input electrical signal with a bit rate f₀ inputdirectly into the electrical signal electrical sampling device 12, forexample.

[0031] Furthermore, to not have the repetition frequency f₁ of thesampling clock follow the bit phase of the optical signal (or theelectrical signal) using clock extraction or the like, means generatinga sampling signal with a repetition frequency f₁ (Hz) independently ofthe data signal. Here, “independently” means that the relationshipbetween the bit phases of the data signal and the sampling signal do notconstantly track each other.

[0032] Moreover, in the reading of a plurality of pieces of sampled datafrom the voltage retaining device 14 by the electrical signal processingdevice 16, the plurality of pieces of the sampled data may be read atonce, or sequentially.

[0033] Second Embodiment

[0034] A second embodiment of a data signal quality evaluation apparatusof the present invention is shown in FIG. 2. This embodiment is inaccordance with the invention as disclosed in claim 10. In the presentembodiment, an optical signal electrical sampling device 22 is used asthe data signal sampling device. In such a case, the input data signalis an optical data signal, and a sampling clock generation device 23 isused as the sampling pulse train generation device. In this case,because a sampled optical signal is obtained by the optical signalelectrical sampling device 22, then in order to convert the sampledsignal into a piece of electrical digital data and store the piece ofthe electrical digital data, it is necessary to perform analog/digitalconversion after performing opto-electric conversion. Consequently, theconstruction shown in FIG. 2 is such that a voltage retaining device 14having an analog/digital conversion function is provided after theopto-electric conversion device 21. In FIG. 2, those structural elementswhich are the same as in FIG. 1 are given the same reference numerals.The operation of the present embodiment is described below.

[0035] An optical signal with a bit rate of f₀ (bit/s) arrives at theoptical signal electrical sampling device 22. In addition, a samplingclock is generated by the sampling clock generation device 23 at arepetition frequency of f₁ (Hz) (f₁=(n/m)f₀+a or f₁=(n/m)f₀−a, where nand m are natural numbers and a is the offset frequency), and arrives atthe optical signal electrical sampling device 22. Here, devices such aselectrical short pulse generation by means of a combination of asynthesized signal generator and a comb generator can be used as thesampling clock generation device 23. In addition, it is preferable thatthe repetition frequency f₁ of the sampling clock is high-speed, in theMHz to GHz region. Furthermore, it is preferable that the frequency bandof the comb generator is extended to approximately four times the bitrate f₀ of the optical signal, and that the pulse width of theelectrical short pulse is set to approximately the time width asdetermined by the Fourier transform of the frequency band of the combgenerator. Moreover, an electric amplifier could be used before or afterthe comb generator as needed. Additionally, a baseband clipper could beused after the comb generator as needed.

[0036] In the optical signal electrical sampling device 22, the opticalsignal is sampled in accordance with the sampling clock to obtain asampled optical signal with a bit rate f₁. In the optical signalelectrical sampling device 22, a gate operation performed by anelectro-absorption optical modulator, or the like, may be used. Here,the transmission band of the optical signal of the optical signalelectrical sampling device 22 is preferably close to the optical signalbit rate f₀. Furthermore, the sampling gate width of the optical signalelectrical sampling device 22 is preferably equal to or less than ¼ orthereabouts of the time determined by the reciprocal of the bit rate f₀of the optical signal. The sampled optical signal is then converted intoa sampled electrical signal by the opto-electric conversion device 21.

[0037] The voltage retaining device 14 performs analog/digitalconversion (A/D conversion) of the sampled electrical signal accordingto trigger signals from the trigger signal generation device 15indicating the start of data acquisition, and performs a temporarystorage operation. The voltage retaining device 14 stores a plurality ofpieces of sampled data for the time until a trigger signal indicatingthe finish of data acquisition is sent from the trigger signalgeneration device 15, and outputs the plurality of pieces of the sampleddata according to a trigger signal from an external source. Here forexample, a high speed A/D conversion circuit which comprises anelectrical buffer memory with a capacity equal to or greater than akilobyte and which has the function of MHz to GHz sampling could beused. After data acquisition has been performed for a fixed period oftime and a plurality of pieces of sampled data have been stored in thevoltage retaining device 14, a trigger signal indicating the start ofdata read is sent from the trigger signal generation device 15 to theelectrical signal processing device 16. Then the electrical signalprocessing device 16 reads the plurality of pieces of the sampled datafrom the voltage retaining device 14 according to the trigger signalindicating the start of data read, obtains the signal eye-diagram fromthe plurality of pieces of the sampled data, and evaluates the opticalsignal quality parameters.

[0038] Third embodiment

[0039] A third embodiment of a data signal quality evaluation apparatusof the present invention is shown in FIG. 3. This embodiment is inaccordance with the invention as disclosed in claim 10. In the presentembodiment, an optical signal optical sampling device 32 is used as thedata signal sampling device. In such a case, the input data signal is anoptical data signal, and a sampling optical pulse train generationdevice 33 is used as the sampling pulse train generation device. In thiscase, because a sampled optical signal is obtained by the optical signaloptical sampling device 32, then in order to convert the sampled signalinto a piece of electrical digital data and store the piece ofelectrical digital data, it is necessary to perform analog/digitalconversion after performing opto-electric conversion. Consequently, theconstruction shown in FIG. 3 is such that a voltage retaining device 14having an analog/digital conversion function is provided after theopto-electric conversion device 21. In FIG. 3, those structural elementswhich are the same as in FIG. 1 or FIG. 2 are given the same referencenumerals. The operation of the present embodiment is described below.

[0040] An optical signal with a bit rate f₀ (bit/s) arrives at theoptical signal optical sampling device 32. In addition, a samplingoptical pulse train is generated by the sampling optical pulse traingeneration device 33 at a repetition frequency f₁ (Hz) (f₁=(n/m)f₀+a orf₁=(n/m)f₀−a, where n and m are natural numbers and a is the offsetfrequency), and arrives at the optical signal optical sampling device32. Here, the sampling optical pulse train has a pulse widthsufficiently smaller than the time determined by the reciprocal of thebit rate f₀ of the optical signal. Gain switching type laser diodes,fiber ring lasers and mode locking laser diodes and the like can be usedas the sampling optical pulse train generation device 33. Here, it ispreferable that the repetition frequency f₁ of the sampling opticalpulse train is high-speed, in the MHz to GHz region. Furthermore, thepulse width of the sampling optical pulse train is preferably equal toor less than ¼ of the time determined by the reciprocal of the bit ratef₀ of the optical signal.

[0041] The optical signal optical sampling device 32 samples the opticalsignal in accordance with the sampling optical pulse train to obtain asampled optical signal. Here, a nonlinear optical effect between theoptical signal and the sampling optical pulse train can be used for theoptical signal optical sampling device 32, and a nonlinear opticalmedium such as KTP (KTiOPO₄), AANP (2-adamantylamino-5-nitropyridine) orPPLN (Periodically Poled Lithium Niobate) can be used for this purpose.Furthermore, SFG (sum frequency (optical signal) generation), SHG(second-order harmonic (optical signal) generation) or FWM (four wavemixing) can be employed as the nonlinear optical effect.

[0042] The sampled optical signal is converted into a sampled electricalsignal by the opto-electric conversion device 21. The voltage retainingdevice 14 performs analog/digital conversion (A/D conversion) of thesampled electrical signal according to trigger-signals from a triggersignal-generation device 15 indicating te start of data acquisition, andperforms a temporary memory storage operation. The voltage retainingdevice 14 stores a plurality of pieces of sampled data for the timeuntil a trigger signal indicating the finish of data acquisition is sentfrom the trigger signal generation device 15, and outputs the pluralityof pieces of the sampled data according to a trigger signal from anexternal source. Here for example, a high speed A/D conversion circuitwhich comprises an electrical buffer memory with a capacity equal to orgreater than a kilobyte and which has the function of MHz to GHzsampling could be used. After data acquisition has been performed for afixed period of time and a plurality of pieces of sampled data have beenstored in the voltage retaining device 14, a trigger signal indicatingthe start of data reading is sent from the trigger signal generationdevice 15 to the electrical signal processing device 16. Then theelectrical signal processing device 16 reads the plurality of pieces ofthe sampled data from the voltage retaining device 14 according to thetrigger signal indicating the start of data read, obtains the signaleye-diagram from the plurality of pieces of the sampled data, andevaluates the optical signal quality parameters.

[0043] Fourth Embodiment

[0044] In this embodiment, an example of an evaluation procedure for usewhen an accurate value is not known for the signal bit rate f₀ isdescribed. First, if the signal format is known, the signal bit rate canbe assumed to be in the case of SDH, for example, one of either 2.48832Gbit/s, 9.95328 Gbit/s, 39.81312 Gbit/s, . . . . However, to beaccurate, it can be assumed that there is actually deviation of df (Hz)in the signal bit rate, and in the case of SDH, for example, there is anallowance of df=±200 ppm. Assuming actual deviation of df in the bitrate, if the repetition frequency of the sampling clock is determinedbased on, for example, a common measure of 2.48832 Gbit/s, 9.95328Gbit/s and 39.81312 Gbit/s without-taking df into consideration,deviation also occurs in f₁ by an amount attributable to df. If this f₁satisfies f₁ (n/m)f₀±a (where M and m are natural numbers) and (n/m)²q/{k+(n/m) q}f₀≦a <(n/m)²q/{k+(n/m)q−1}f₀ (where k is a natural number),the measurement of an open eye-diagram can be achieved. If fi does notsatisfy these conditions, the measurement of an open eye-diagram can beachieved by sweeping the value of f₁ or sweeping at least one of thevalue of f₁, the value of k, the value of n/m and the value of q so thatikj≦Nsamp and ikj<kz/{2q(k−z)} (where i,j, and Nsamp are naturalnumbers) are satisfied. What n/m, k, q, and z refer to is described indetail in the “display examples of eye-diagrams according to theembodiments” below. This method of sweeping these parameters can beapplied not only to a case where the signal bit rate is known to acertain extent based on the signal format, but also to a case when thesignal bit rate is completely unknown. However, in such a case, thedemand for the variable width of f₁ is greater.

[0045] The condition under which eye-diagram measurement is possiblewithout sweeping the value of f₁ is f₁≧(2·f₀ ·Nsamp ·|df|)_(1/2),Therefore, if f₀=9.95328 Gbit/s, df=200 ppm and Nsamp=250, for example,then f₀≧the order of 1 GHz, and by using a high speed sampling rate,eye-diagram measurement is possible even in a case where the signal bitrate cannot be known accurately.

[0046] Quality Evaluation Examples According to the Embodiments

[0047] Examples of quality evaluation according to the first embodimentto the fourth embodiment of the present invention are described withreference to FIG. 4 through FIG. 8. Each diagram shows an example of atypical synchronous eye-diagram and an example of the evaluationparameters.

[0048]FIG. 4 shows an example of an eye-diagram-for an NRZsignal(Non-Return-to-Zero signal). An amplitude histogram is shown for acase in which the time window of di is set centered about the time whenthe eye opening is the largest in the intensity domain. The means μ₁ andμ₀ and the standard deviations σ₁ and σ₀ are evaluated for thedistributions of the mark and space levels, respectively.

[0049] Furthermore, the Q-factor determined by

Q=|μ ₁−μ₀|/σ₁+σ₀

[0050] can be used as an evaluation parameter. Here, |μ₁−μ₀|is theabsolute value of the difference between the means μ₁ and μ₀.

[0051] The bit error rate can be estimated from the Q-factor, usingBER=erfc (Q). Here, BER is the Bit Error Rate, and erfc is thecomplementary error function.

[0052]FIG. 5 shows an example of an eye-diagram for an RZ signal(Return-to-Zero signal). The means μ₁, μ₀, the standard deviations σ₁,σ₀ and the Q-factor are evaluated in the same manner as described abovefor an NRZ signal.

[0053]FIG. 6 shows an example of an eye-diagram of an NRZ signal. Anamplitude histogram is shown in which the time window of dt is set withthe cross-point of the eye-diagram as its center. The means μ₁, μ₀ andμ_(cross) are evaluated for the distributions of the mark and spacelevels and the frequency distribution near the cross-point,respectively.

[0054] As an example, R_(cross)=|μ_(cross)−μ₀|/|μ₁−μ₀ indicates thedeviation in the amplitude intensity at the cross-point, and istherefore a parameter for evaluating the effect of pulse broadeningcaused by wavelength dispersion or the like.

[0055]FIG. 7 shows an example of an eye-diagram for an NRZ signal. Thisdiagram shows a time histogram of the section defined by the time widthdt and the intensity width dI, and in this example the standarddeviation a is evaluated. σ₁ is a parameter for evaluating the effectsof jitter.

[0056]FIG. 8 is an example of an eye-diagram for an RZ signal. Thediagram on the left shows an amplitude histogram in which the timewindow of dt_(a) is set with the time when the eye opening in theintensity domain is the largest as its center, and the means μ₁ and μ₀are evaluated for the distributions of the mark and space levels,respectively. The diagram at the top shows a time histogram of thesection defined by the time width dt and the intensity width dI,centered on the value of |μ₁−μ₀/2, and in this example the standarddeviation σ₁ and the difference between the means T_(FWHM) areevaluated. σ₁ is a parameter for evaluating the effects of jitter.T_(FWHM) indicates the full width at half maximum of the RZ pulse, whichis a parameter for evaluating the pulse broadening caused by wavelengthdispersion.

[0057] In addition, by setting the windows of the time width dt and thefrequency width dI arbitrarily, it is possible to evaluate thedegradation caused by PMD (polarization mode dispersion) and the like.

[0058] Display Examples of Eye-Diagrams According to the Embodiments

[0059] Regarding the display of eye-diagrams, a plurality of pieces ofsampled data can be displayed on a display device as is, in the order inwhich they were sampled. In such a case, instead of arranging everysampled point in a time series, the sampled points may be superposedfrom time zero at an interval of a certain period. An eye-diagram can bedisplayed by repeating this process for every sampled point.

[0060] The superposition period is described below. Here, a case isdescribed in which when the bit rate of the data signal is f₀(bit/s),and the repetition frequency f₁(Hz) of the sampling is represented byf₁(n/m)f₀+a, or f₁=(n/m)f₀−a (where, Land in are natural numbers, and ais the offset frequency), a satisfies the condition${\frac{( \frac{n}{m} )^{2}q}{k + {( \frac{n}{m} )q}}f_{0}} \leq a < {\frac{( \frac{n}{m} )^{2}q}{k - 1 + {( \frac{n}{m} )q}}f_{0}}$

[0061] (where k and q and natural numbers)

[0062] Here, n/m is a value relating to the ratio between f₀ an f₁, andif n/m is {fraction (1/100)} and f ₀ is 10 (Gbit/s), for example, thenf₁ is approximately 100 (MHz), showing that the sampling frequency issuch that one sampled point is obtained for every approximately 100 bitsof the data signal. Furthermore, k is a value relating to thesuperposition period, indicating that sampled points are superposed inunits of k. Furthermore, q is a value indicating how many bits of thedata signal are reproduced when k sampled points are arranged in a timeseries. As an example, plotting examples of the points P1 to P8 eachcorresponding with a piece of sampled data are described below for acase where f₁=(n/m)−a, with reference to FIG. 9A to FIG. 9D. FIG. 9A isa diagram showing the waveform of a data signal (although only points P1to P5 are shown in FIG. 9A), and FIG. 9B to FIG. 9D are diagrams showingplotting examples thereof. Furthermore, in this case the variablessatisfy n/m=1, k=4 and q=1.

[0063] In the case above, the value of the offset frequency a(=±(f₁−f₀)) is within the range of ({fraction (1/5)})f ₀≦a<({fraction(1/4)})f ₀. In other words, ¼/f₀≦1f₁−1/f₀<⅓/f₀ is satisfied, and Δt(=1/f₁−1/f₀) is set to a value greater than {fraction (1/4)} and lessthan ⅓ of one timeslot which is the reciprocal of f₀. The waveformwithin one timeslot is reproduced by arranging points P1 to P4 in order(FIG. 9B).

[0064] In this example, point P5 is not plotted in a position followingpoint P4, and is instead plotted after returning to time zero. Here,there are two possible superposition methods.

[0065] (1) The first-hsuperposition5methodinvolves-aligning the timeposition of poit P5 with the time position of point P1, as shown in FIG.9C. When the time position of point P5 is aligned to the time positionof point PI, the second superposed waveform presents slight temporaldeviation relative to the first waveform. In superposing the third andthen fourth waveforms in the same manner, the degree of deviationincreases gradually, and consequently the eye tends towards closing asthe number of superposed waveforms increases. The only informationrequired to realize this superposition is the value of n/m. Because thesampling clock can be set locally, k can be determined arbitrarilywithin the range of natural numbers, and the superposition period isdetermined according to k. k is a natural number, and it can be saidthat a larger value is preferable for the reproduction of a complicatedwaveform.

[0066] (2) The second superposition method involves aligning the timeposition of point P5 to a multiple of 1/f₀ as shown in FIG. 9D. If thetime position of point P5 is aligned to a multiple of 1/f₀, the secondsuperposed waveform overlaps the first waveform. However. it isnecessary to know the absolute value of f₀.

[0067] First, the deviation occurring when the time position of point P5is aligned to the same time position as point P1 in method (1) isestimated. If a=(¼)f₀, then point P5 is aligned to point P1 at a periodof 1/f₀, and consequently if superposition is performed in units of fourpoints (or if superposition is performed based on a multiple of four),even if the superposition of waveforms is continued infinitely, a cleaneye-diagram can be obtained. However, in this case, as is apparent fromthe equation above used to define the range of a, a deviates slightlyfrom (¼)f₀.

[0068] Here, assuming that z is a real number which satisfies k−1<z<k,then$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

[0069] and because in the current case n/m=1 and q=1, z is a real numberwhich satisfies 3<z<4, and therefore a={1/(z+1)}f₀. Performing thecalculations based on these facts shows that in comparison with a casewhere a (¼)f₀, the size ΔT of the deviation which occurs whensuperposing waveforms is ΔT=(4−z)/(zf₀). In other words, if the periodof the superposition is ik (where i is a natural number, in this examplei=l), then ΔT=q (k−z) i/(zf₀). In other words, as the waveforms aresuperposed a second and a third time, and so on, each waveform deviatesby an additional AT in the time domain. Once the total deviation equalshalf the size of a timeslot which is the reciprocal of f₀, theeye-diagram is completely closed, and as such this is the upper limitfor deviation. If the number of sampled points to be measured at a timeis deemed Nsamp, and the number of superposition is deemed I, thenikj≦Nsamp. Accordingly, if the total accumulated deviation is deemed Sum[ΔT], Sum [ΔT] is expressed as${{Sum}\lbrack {\Delta \quad T} \rbrack} = \frac{{q( {k - z} )}i\quad j}{z\quad f_{0}}$

[0070] Because a condition for enabling eye opening evaluation is thatthis value is equal to or less than half of I/f₀, if the number ofsampled points is within a range which satisfies$\quad {\frac{( {k - z} )i\quad j\quad q}{z\quad f_{0}} \leq \frac{1}{2f_{0}}}$that  is$\quad {{ikj} \leq \frac{z\quad k}{2{q( {k - z} )}}}$

[0071] then the eye opening cm be-evaluated even if a local clock isused.

[0072] In other words, when the value of a is$\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}$

[0073] (where z is a real number which satisfies k−1<z≦k)

[0074] and Nsamp pieces of sampled data are displayed in the order ofmeasurement at a time interval of dt=1/(zf₀) in the time domain, then inthe case where a signal eye-diagram is obtained and data signal waveformmeasurement and quality evaluation are performed by, deeming the timeposition of the first piece of the sampled data t=0, displaying theplurality of pieces of the sampled data in a superposed manner byreturning the time position to 0 each time t=p/f₀ (where p is a naturalnumber), if the number of superposition is deemed j (where j is anatural number), data signal quality evaluation can be performed for theentire number of pieces of the sampled data Nsamp, by satisfyingpkj≦Nsamp.

[0075] To further expand on the above-described process, evaluation canbe performed after repeating i times the process of acquiring Nsanppieces of data and displaying an eye-diagram as described above, andthen superposing j eye-diagrams based on the point where the eye openingis largest. Using this process, the effective number of sampled pointsused in the evaluation can be increased, thereby further reducing theindeterminacy of the evaluation.

[0076] The entire contents of Priority Documents 2002-139006 and2003-118299 are incorporated herein by reference.

What is claimed is:
 1. A data signal quality evaluation method,comprising the steps of: after repeating N times (where N is a naturalnumber) a process in which an input optical or electrical data signalwith a bit rate of f₀ (bit/s) is sampled using an optical or electricalsampling pulse train, which is generated independently of the datasignal, and which has a repetition frequency f₁ (Hz) which differs fromthe bit rate f₀ (bit/s), a thus obtained optical or electrical sampledsignal is converted into a piece of electrical digital data, and thepiece of the electrical digital data is stored in a buffer, reading Npieces of the electrical digital data stored in the buffer at once orsequentially and performing electrical signal processing of the N piecesof the electrical digital data, to obtain a signal eye-diagram and toperform data signal waveform measurement and quality evaluation.
 2. Adata signal quality evaluation method according to claim 1, wherein therepetition frequency f₁ (Hz) of sampling satisfies f₁=(n/m)f₀±a (where nand m are natural numbers), and a range of variable a is${\frac{( \frac{n}{m} )^{2}q}{k + {( \frac{n}{m} )q}}f_{0}} \leq a < {\frac{( \frac{n}{m} )^{2}q}{k - 1 + {( \frac{n}{m} )q}}f_{0}}$

(where k and q are natural numbers).
 3. A data signal quality evaluationmethod according to claim 2, wherein when the value of the variable a isexpressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and pieces of sampleddata are displayed in the order of measurement at a time interval ofdt=1/(zf₀) in the time domain, the time positions of every ik piece(where i is a natural number) of the sampled data counting from thefirst piece of the sampled data are superposed and the pieces of thesuperposed sampled data are displayed, to obtain the signal eye-diagramand to perform the data signal waveform measurement and the qualityevaluation, and wherein when the number of times superposition isperformed is j (where j is a natural number), ikj≦Nsamp is satisfied forthe total number of pieces of the sampled data Nsamp.
 4. A data signalquality evaluation method according claim 3, wherein the variables i andk are values which satisfy ikj≦Nsamp, and satisfy${i\quad j\quad k} \leq {\frac{k\quad z}{2{q( {k - z} )}}.}$


5. A data signal quality evaluation method according to claim 4, whereinby repeating a plurality of times a process of reading Nsamp pieces ofthe sampled data stored in the buffer at once or sequentially andperforming electrical signal processing to obtain the signaleye-diagram, and superposing signal eye-diagrams so that the eyeopenings thereof match temporally, the total number of pieces of thesampled data which constitute the signal eye-diagrams is increased, andthe data signal waveform measurement and the quality evaluation areperformed.
 6. A data signal quality evaluation method according to claim5, wherein either one of an amplitude histogram and a time histogram,determined from a sampled data distribution obtained by dividing anobtained eye-diagram in the intensity domain and the time domain,respectively, is used as a data signal quality parameter.
 7. A datasignal quality evaluation method according to claim 2, wherein when thevalue of the variable a is expressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and pieces of sampleddata are displayed in the order of measurement at a time interval ofdt=1/(zf₀) in the time domain, the time position of the first piece ofthe sampled data is deemed, t=0 and the pieces of the sampled data aredisplayed in a superposed manner by returning the time position to zeroeach time t=p/f₀ (where p is a natural number), to obtain the signaleye-diagram and to perform the data signal waveform measurement and thequality evaluation, and wherein when the number of times superpositionis performed is j (where j is a natural number), pkj≦Nsamp is satisfiedfor the total number of pieces of the sampled data Nsamp.
 8. A datasignal quality evaluation method according to claim 7, wherein byrepeating a plurality of times a process of reading Nsamp pieces of thesampled data stored in the buffer at once or sequentially and performingelectrical signal processing to obtain the signal eye-diagram, andsuperposing signal eye-diagrams so that the eye openings thereof matchtemporally, the total number of pieces of the sampled data whichconstitute the signal eye-diagrams is increased, and the data signalwaveform measurement and the quality evaluation are performed.
 9. A datasignal quality evaluation method according to claim 8, wherein eitherone of an amplitude histogram and a time histogram, determined from asampled data distribution obtained by dividing an obtained eye-diagramin the intensity domain and the time domain, respectively, is used as adata signal quality parameter.
 10. A data signal quality evaluationapparatus, comprising: a sampling pulse train generation device whichgenerates an optical or electrical sampling pulse train, independentlyof an input optical or electrical data signal with a bit rate of f₀(bit/s), and which has a repetition frequency f₁ (Hz) which differs fromthe bit rate f₀ (bit/s); a data signal sampling device which samples thedata signal with the bit rate f₀ (bit/s) in accordance with the samplingpulse train to obtain a sampled signal; a voltage retaining device whichconverts the sampled signal which is an optical or electrical sampledsignal obtained by means of the data signal sampling device into a pieceof electrical digital data, and stores a plurality of pieces of theelectrical digital data; an electrical signal processing device whichreads the plurality of pieces of the electrical digital data stored inthe voltage retaining device at once or sequentially to obtain a signaleye-diagram and to evaluate optical data signal quality parameters; anda trigger signal generation device which applies triggers indicating thestart/finish of data acquisition to the voltage retaining device, andapplies triggers indicating the start/finish of data read to theelectrical signal processing device.
 11. A data signal qualityevaluation apparatus according to claim 10, wherein the repetitionfrequency f₁ (Hz) of sampling satisfies f₁=(n/m)f₀+a (where n and m arenatural numbers), and a range of variable a is${\frac{( \frac{n}{m} )^{2}q}{k + {( \frac{n}{m} )q}}f_{0}} \leq a < {\frac{( \frac{n}{m} )^{2}q}{k - 1 + {( \frac{n}{m} )q}}f_{0}}$

(where k and q are natural numbers).
 12. A data signal qualityevaluation apparatus according to claim 11, wherein the sampling pulsetrain generation device has a function which renders the repetitionfrequency f₁ (Hz) of the generated sampling pulse train variable, andwhich when an accurate value for the bit rate f₀ (bit/s) is unknown,sweeps the value of f₁ so that f₁=(n/m)f₀ ±a (where n and m are naturalnumbers) and (n/m)²q/{k+(n/m)q}f₀≦a <(n/m)²q/{k+(n/m)q−1}f₀ (where k isa natural number) are satisfied.
 13. A data signal quality evaluationapparatus according to claim 12, wherein when the value of the variablea is expressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and the pieces ofsampled data are displayed in the order of measurement at a timeinterval of dt=1/(zf₀) in the time domain, the time positions of everyik piece (where i is a natural number) of the sampled data counting fromthe first piece of the sampled data are superposed and the pieces of thesuperposed sampled data are displayed, to obtain the signal eye-diagramand to perform data signal waveform measurement and quality evaluation,and wherein when the number of times superposition is performed is j(where j is a natural number), ikj≦Nsamp is satisfied for the totalnumber of pieces of the sampled data Nsamp.
 14. A data signal qualityevaluation apparatus according to claim 13, wherein the variables i, kare values which satisfy ikj<Nsamp, and satisfy${ijk} \leq {\frac{kz}{2{q( {k - z} )}}.}$


15. A data signal quality evaluation apparatus according to claim 14,wherein when an accurate value for the, bit rate f₀ (bit/s) is unknown,the electrical signal processing device sweeps at least one of the valueof k, the value of n/m and the value of q so that the repetitionfrequency f₁ (Hz) of sampling satisfies f₁=(n/m)f₀±a (where n and m arenatural numbers) and (n/m)²q/{k+(n/m) q}f₀≦a<(n/m)²q/{k+(n/m)q−1}f₀(where k is a natural number), and ikj≦Nsamp and ikj≦kz/{2q (k−z)}(where i, j, and Nsamp are natural numbers) are satisfied.
 16. A datasignal quality evaluation apparatus according to claim 15, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number-of-piecesof-the sampled data which constitute the signal eye-diagrams for thedata signal waveform measurement and the quality evaluation is increasedby superposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 17. A data signal quality evaluation apparatusaccording to claim 16, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 18. A data signal qualityevaluation apparatus according to claim 15, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and the time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain, respectively.
 19. A datasignal quality evaluation apparatus according to claim 14, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 20. A data signal quality evaluation apparatusaccording to claim 19, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 21. A data signal qualityevaluation apparatus according to of claim 14, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and the time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain, respectively.
 22. A datasignal quality evaluation apparatus according to claim 13, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal, eye-diagrams so that the eye openings thereofmatch temporally.
 23. A data signal quality evaluation apparatusaccording to claim 22, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 24. A data signal qualityevaluation apparatus according to claim 13, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and the time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain, respectively.
 25. A datasignal quality evaluation apparatus according to claim 12, wherein whenthe value of the variable a is expressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and pieces of sampleddata are displayed in the order of measurement at a time interval ofdt=1/(zf₀) in the time domain, the time position of the first piece ofthe sampled data is deemed t=0 and the pieces of the sampled data aredisplayed in a superposed manner by returning the time position to zeroeach time t=p/f₀ (where p is a natural number), to obtain a signaleye-diagram and to perform data signal waveform measurement and qualityevaluation, and wherein when the number of times superposition isperformed isj (where j is a natural number), pkj≦Nsamp is satisfied forthe total number of pieces of the sampled data Nsamp.
 26. A data signalquality evaluation apparatus according to claim 25, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 27. A data signal quality evaluation apparatusaccording to claim 26, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter- and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 28. A data signal qualityevaluation apparatus according to claim 25, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and the time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain, respectively.
 29. A datasignal quality evaluation apparatus according to claim 11, wherein whenthe value of the variable a is expressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and pieces of sampleddata are displayed in the order of measurement at a time interval ofdt=1/(zf₀) in the. time domain, the time positions of every ik piece(where i is a natural number) of the sampled data counting from thefirst piece of the sampled data are superposed and the pieces of thesuperposed sampled data are displayed, to obtain the signal eye-diagramand to perform data signal waveform measurement and quality evaluation,and wherein when the number of times superposition is performed j (wherej is a natural number), ikj≦Nsamp is satisfied for the total number ofpieces of the sampled data Nsamp.
 30. A data signal quality evaluationapparatus according to claim 29, wherein the variables i, k are valueswhich satisfy ikj≦Nsamp, and satisfy${ijk} \leq {\frac{kz}{2{q( {k - z} )}}.}$


31. A data signal quality evaluation apparatus according to claim 30,wherein when an accurate value for the bit rate f₀ (bit/s) is unknown,the electrical signal processing device sweeps at least one of the valueof k, the value of n/m and the value of q so that the repetitionfrequency f₁ (Hz) of sampling satisfies f₁=(n/m)f₀±a (where n and m arenatural numbers) and (n/m)²q/{k+(n/m)q}f₀≦a<(n/m)²q/{k+(n/m)q−1} f₀(where k is a natural number), and ikj≦Nsamp and ikj≦kz/{2q (k−z)}(where i,j, and Nsamp are natural numbers) are satisfied.
 32. A datasignal quality evaluation apparatus according to claim 31, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 33. A data signal quality evaluation apparatusaccording to claim
 32. wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 34. A data signal qualityevaluation apparatus according to claim 31, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and-the-time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain, respectively.
 35. A datasignal quality evaluation apparatus according to claim 30, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 36. A data signal quality evaluation apparatusaccording to claim 35, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 37. A data signal qualityevaluation apparatus according to claim 30, wherein theelectrical-signal processing device comprises at least one of anamplitude histogram evaluation section which determines an amplitudehistogram as a data signal quality parameter and a time histogramevaluation section which determines a time histogram as a data signalquality parameter, and the amplitude histogram and the time histogramare determined from a sampled data distribution obtained by dividing asignal eye-diagram in the amplitude domain and the time domain,respectively.
 38. A data signal quality evaluation apparatus accordingto claim 29, wherein the electrical signal processing device repeats aplurality of times a process of reading the Nsamp pieces of the sampleddata stored in the voltage retaining device at once or sequentially andobtaining a signal eye-diagram, and wherein the data signal qualityevaluation apparatus further comprises an eye opening evaluation sectionwhich evaluates the eye-opening of signal eye-diagrams, and the totalnumber of pieces of the sampled data which constitute the signaleye-diagrams for the data signal waveform measurement and the qualityevaluation is increased by superposing the signal eye-diagrams so thatthe eye openings thereof match temporally.
 39. A data signal qualityevaluation apparatus according to claim 38, wherein the electricalsignal processing device comprises at least one of an amplitudehistogram evaluation section which determines an amplitude histogram asa data signal quality parameter and a time histogram evaluation sectionwhich determines a time histogram as a data signal quality parameter,and the amplitude histogram and the time histogram are determined from asampled data distribution obtained by dividing a signal eye-diagram inthe amplitude domain and the time domain. respectively.
 40. A datasignal quality evaluation apparatus according to claim 29, wherein theelectrical signal processing device comprises at least one of anamplitude histogram evaluation section which determines an amplitudehistogram as a data signal quality parameter and a time histogramevaluation section which determines a time histogram as a data signalquality parameter, and the amplitude histogram and the time histogramare determined from a sampled data distribution obtained by dividing asignal eye-diagram in the amplitude domain and the time domain,respectively.
 41. A data signal quality evaluation apparatus accordingto claim 1 wherein when the value of the variable a is expressed as$a = {\frac{( \frac{n}{m} )^{2}q}{z + {( \frac{n}{m} )q}}f_{0}}$

(where z is a real number which satisfies k−1<z≦k) and pieces of sampleddata are displayed in the order of measurement at a time interval ofdt=1/(zf₀) in the time domain, the time position of the first piece ofthe sampled data is deemed t=0 and the pieces of the. sampled data aredisplayed in a superposed manner by returning the time position to zeroeach time t=p/f₀ (where p is a natural number), to obtain a signaleye-diagram and to perform data signal waveform measurement and qualityevaluation, and wherein when the number of times superposition isperformed is j (where j is a natural number), pkj≦Nsamp is satisfied forthe total number of pieces of the sampled data Nsamp.
 42. A data signalquality evaluation apparatus according to claim 41, wherein theelectrical signal processing device repeats a plurality of times aprocess of reading the Nsamp pieces of the sampled data stored in thevoltage retaining device at once or sequentially and obtaining a signaleye-diagram, and wherein the data signal quality evaluation apparatusfurther comprises an eye opening evaluation section which evaluates theeye-opening of signal eye-diagrams, and the total number of pieces ofthe sampled data which constitute the signal eye-diagrams for the datasignal waveform measurement and the quality evaluation is increased bysuperposing the signal eye-diagrams so that the eye openings thereofmatch temporally.
 43. A data signal quality evaluation apparatusaccording to claim 42, wherein the electrical signal processing devicecomprises at least one of an amplitude histogram evaluation sectionwhich determines an amplitude histogram as a data signal qualityparameter and a time histogram evaluation section which determines atime histogram as a data signal quality parameter, and the amplitudehistogram and the time histogram are determined from a sampled datadistribution obtained by dividing a signal eye-diagram in the amplitudedomain and the time domain, respectively.
 44. A data signal qualityevaluation apparatus according to claim 41, wherein theelectrical-signal processing device comprises at least one of anamplitude histogram evaluation section which determines an amplitudehistogram as a data signal quality parameter and a time histogramevaluation section which determines a time histogram as a data signalquality parameter, and the amplitude histogram and the time histogramare determined from a sampled data distribution obtained by dividing asignal eye-diagram in the amplitude domain and the time domain,respectively.
 45. A data signal quality evaluation apparatus accordingto claim 11, wherein the electrical signal processing device comprisesat least one of an amplitude histogram evaluation section whichdetermines an amplitude histogram as a data signal quality parameter anda time histogram evaluation section which determines a time histogram asa data signal quality parameter, and the amplitude histogram and thetime histogram are determined from a sampled data distribution obtainedby dividing a signal eye-diagram in the amplitude domain and the timedomain, respectively.